This invention relates to a new co-catalyst composition which comprises mixed organoaluminum compounds and certain Lewis bases dissolved in at least one aromatic solvent. The co-catalyst system in combination with a transition metal catalyst is useful for the production of polyolefins in accordance with a high efficiency process for producing polyolefins. The invention, therefore, further relates to a catalyst system comprising the co-catalyst composition and to the process of polymerizing one or more olefins in the presence of the catalyst system.